Rising energy costs and stretched power grids as well as a desire for energy independence has sparked a recent surge in the use of solar panels (photovoltaic) to make electricity. Currently, over 90% of solar cells in the market use silicon. However, the lack of an intermediate grade of silicon has hampered the growth of the silicon solar industry. Until recent years, the total demand of silicon for solar cells was small enough to be sufficient sustained by left over scrap silicon from the electronics and semiconductor industry. The new demand, however, has completely outstripped such source of silicon.
Currently, there are two grades of silicon. There is a metallurgical grade (MG) used by the steel and metals industry as an alloying material. This material is made from relatively crude materials (sand and coal or coke) and yields a cheap source of silicon at about 98-99% purity. This is not pure enough for solar grade (SoG) silicon that requires about 99.999% (5, 9's) or 99.9999% (6, 9's). Some companies (such as Elkem) produce higher purity MG silicon by using aluminum instead of carbon as the reducing agent. This material is often used to make electronic or semiconductor grade silicon which is better than 99.999999% (8, 9's) pure.
The method to make 8, 9's silicon is called the Siemens process, which uses MG silicon as a starting material. The process is very capital intensive and expensive to run and causes Siemen's silicon to be very expensive. Solar cells require very large area of silicon to absorb sunlight so that the cost associated with 8, 9' silicon in solar cells is prohibitive. Silicon produced as waste material during the preparation of the 8, 9's silicon often meets the SoG silicon specifications. However, the electronic industry only produces about 4,000 tons per year of such scrap silicon, which cannot meet the current demand for solar silicon, e.g., over 10,000 tons per year.
Much effort has been expended to try to upgrade MG silicon to SoG silicon. The Siemens process does this chemically by reacting MG silicon with HCl at high temperature. This produces a family of chloro-silanes and other impurities that are then rigorously distilled and purified until only a very pure stream of trichlorosilane remains. This material with hydrogen added is decomposed over high purity silicon heated silicon to decompose mixture to pure (8, 9's) silicon and HCl.
However, solar cells can be made with silicon of a lesser purity. If a specific process aimed at the 6, 9's purity level were developed for SoG silicon then the solar industry could resume its growth while maintaining a competitive edge for electrical generation costs.
Much effort has been put into starting with MG silicon and upgrading it. The Siemens process does this chemically. Many attempts have been made to use pyrometallurgical processes. However, dealing with molten silicon is difficult and the number of selective tools for purification is few. These tools are primarily, gas reactions, fluxing with solid or molten materials and various methods of direction solidification. All of these methods have their limitations and to date no combination of these methods has produced a viable commercial method that is used by any manufacturer of silicon. The one partial success is the HEM (Heat Exchanger Method) which is a directional solidification method used to increase the purity of bulk silicon. However, this method is not useful for upgrading MG silicon to SoG silicon. At this time the largest furnaces available produce about 200 kilograms of useable silicon every 50 to 60 hours. The method is slow and consumes much energy. Further, the technique depends on materials (being removed) having a partition coefficient significantly less than 1 (typically below 0.1 to be effective). While many materials do have low partition coefficients for the solubility difference in molten verses solid silicon, this method removes many impurities. However, two materials, boron and phosphorus, are particularly deleterious to solar cells and also have high partition coefficient (0.8 for boron) and 0.35 for phosphorus. Thus, the HEM method (a directional solidification method) is not a suitable way to purify silicon if these contaminants are present in an amount above the final desired tolerable limits. Almost all MG grade silicon has boron contents (typically >100 ppm) well above the requirements of a few part per million or less. For a high quality photovoltaic cells, silicon having a boron content of about 1 ppm or less is often required.
In summary, there is no economical source of solar grade silicon. Metallurgical silicon is too impure and semiconductor silicon is too expensive.